Packet Size: Network MTU Vs. Maximum TCP

Too low or too high TCP packet size can adversely affect performance

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The maximum transmission unit (MTU) is the maximum size of a single data unit that can be transmitted over a digital communications network. Higher-level network protocols, like TCP/IP, can be configured with a maximum packet size, which is a parameter independent of the physical layer MTU over which TCP/IP runs. While its possible to set the maximum TCP packet size to almost any value, it should never exceed the network's MTU.

Some network devices incorrectly use these terms interchangeably. For example, on some home broadband routers, the parameter called MTU is, in fact, the maximum TCP packet size.

Network MTU Size
  • Limited by network hardware.

  • Cannot be adjusted without hardware changes.

  • Should never be lower than the maximum TCP packet size.

Maximum TCP Packet Size
  • Can be set to any value.

  • Can be adjusted through your operating system.

  • Should never be higher than the MTU.

MTU Size

The MTU size is a property of a physical network interface and is usually measured in bytes. The MTU for Ethernet, for example, is 1500 bytes. Some types of networks, such as token rings, have larger MTUs, and some networks have smaller MTUs, but the value is fixed for each physical technology.

A larger the MTU means that more data can fit into fewer packets, which generally allows for faster and more efficient transmission. However, if a communications error occurs, the packet will take longer to re-transmit. Since larger packets are more prone to corruption and delays, a smaller MTU can actually improve network latency.

Maximum TCP Packet Size

In Microsoft Windows, the maximum packet size for protocols such as TCP can be set in the Windows Registry. If this value is set too low, streams of network traffic are broken up into a relatively large number of small packets, which adversely affects performance. Xbox Live, for example, requires the value of the packet size to be at least 1365 bytes.

If the maximum TCP packet size is set too high, it exceeds the network's physical MTU and degrades performance by requiring that each packet be subdivided into smaller ones. This process is called fragmentation. Microsoft Windows computers default to a maximum TCP packet size of 1500 bytes for broadband connections and 576 bytes for dial-up connections to avoid exceeding the MTU.

MTU and Max TCP Related Problems

Ethernet's MTU of 1500 bytes limits the size of packets that traverse it. Sending a packet that is larger than the maximum transmission window for Ethernet is called jabbering. If unaddressed, jabbering can disrupt a network. Usually, jabber is detected by repeater hubs or network switches that are designed to do so. The simplest way to prevent jabber is to set the maximum size of a TCP packet to no more than 1500 bytes.

In theory, the limitation of TCP packet size is 64K (65,525 bytes), which is much larger than you'll ever use. Nonetheless, performance problems may also arise if the TCP maximum transmission setting on the home broadband router differs from the setting on individual devices connected to it.